Unfortunately there’s not really a way for Let’s Encrypt to know whether you are/aren’t using a given certificate. For that reason we send expiration warning emails for all certificates issued. If you do not renew the certificates with the unused domains then the emails will cease once the certificate has expired.

netholics:

I want to change/remove this email for notification at all, because it was for testing purposes only.

This depends on the ACME client you are using to communicate with Let’s Encrypt. Certbot (as one example) has a --update-registration flag that can be used to change the email address associated with a registration.